Tire with a specified two-ply steel-aramid belt package

ABSTRACT

The tire of this invention has a belt package with a combination of a steel refinforced inner belt ply and an aromatic polyamide reinforced outer belt ply. The breaking strength, tensile modulus, secant modulus and other material properties for the reinforcing members in each belt ply are defined. The strength per unit width of the aromatic polyamide reinforced ply is made to be within zero to 30 percent greater than the strength per unit width of the steel reinforced belt ply. The tire of this invention has angles of the aromatic polyamide reinforcing members from the mid-plane greater than the angles of the steel reinforcing members from the same mid-plane by 1 to 8 degrees. High impact endurance values are disclosed to be a function of the tensile modulus, breaking strengths and belt ply widths for each ply as well as a function of the composite shear stiffness values.

This is a division of Ser. No. 08/308,909, Sep. 20, 1994, now U.S. Pat.No. 5,662,751.

BACKGROUND OF THE INVENTION

1. Technical Field

The subject matter of this invention relates to radial pneumatic tiresfor vehicles and particularly to the design and configuration of animproved belt package.

2. Discussion of the Art

The modern day tire must be durable, perform well and be resistant towear, as well as light in weight and economical to construct. Theability of vehicles to perform at high rates of speed and to haveexcellent cornering stability remains a desirable feature to theconsumer. The tire must produce the higher forces to achieve thisperformance and at the same time have a smooth ride and absorb shockloads from bumps and holes in the roadway. Another desirable feature ofthe tire is to resist the penetration of water into the belts whichproduces corrosion of the reinforcing materials.

Conventional radial tires use multiple belt plies with steel reinforcingmaterials to achieve vehicle performance. For most applications the tireis sufficiently strong using two steel belt plies. However, these steelreinforced belt plies have a higher mass, do not absorb shock loads verywell and are subjected to higher corrosion. One solution to reduce thetire's weight, improve its shock absorbing ability and to make it moreresistant to corrosion is to use a high modulus textile cord for thereinforcing members in one or more belt plies. An aromatic polyamidecord material has been used to replace steel reinforcing members in sometires with limited success. Improvements in rigidity (stiffness), ridingcomfort, low cost of manufacture, tire weight, corrosion resistance andbelt edge separation have been noted with different designs. However,the need remains to make these improvements to tires even greater;especially to increase their high speed and impact endurance, toincrease their ability to resist corrosion, to help them maintain goodwear rates and to maintain their relatively low weight. Various designsare discussed below.

Tires having multiple belt plies with at least one ply having aromaticpolyamide reinforcing members are disclosed in U.S. Pat. Nos. 3,851,693;4,184,529; 4,184,530; and 4,407,347 as well as French Patent No.2,536,018 and Japanese Patent No. 2-136,304. More than two belt pliesare disclosed by these patents. Such constructions are of interest inthe art as showing the use of an aromatic polyamide material for a beltply reinforcment.

U.S. Pat. No. 4,184,529 teaches a belt package for an off-road tire usedon heavy vehicles. This belt package has three belt plies reinforcedwith steel cables and the possibility to use synthetic textilereinforcing members is mentioned.

The reinforced belt package of U.S. Pat. No. 4,184,530 includes a singleinner belt ply having aromatic polyamide reinforcing members and asingle outer belt ply having metallic reinforcing members; plus two beltedge strips having textile reinforcing members. The angle of thearomatic polyamide belt ply reinforcing members from the mid-plane isless than the angle of the steel belt ply reinforcing members. Otherpatents also show the inner belt having aromatic polyamide reinforcingmaterials. In U.S. Pat. No. 4,407,347 two inner belt plies preferablyhave aromatic polyamide reinforcing members disposed at 15 to 30 degreesplus an outer belt with steel reinforcing members disposed at 60 to 90degrees. The belt package further includes two lateral strips thatadjoin the steel reinforced belt ply at both edges.

Two additional patents which also include a belt package with auxiliarylayers or cover plies are French Patent 2,536,018 and Japanese Patent2-136,304. These patents use an inner belt ply having steel reinforcingmembers a middle belt ply having aromatic polyamide reinforcing membersand an outer cap ply of nylon or polyester reinforcing material at 0 to10 degrees. A wide range of angles is given for the reinforcingmaterials in each of the two inner plies, being from 0 to 35 degrees,and the angles in respective plies are not the same. The claims of2-136,304 are further discussed below.

A number of patents disclose the use of a belt ply comprising aromaticpolyamide reinforcing members where one or more of the belt plies arefolded to give improved endurance of the belt package. These includeU.S. Pat. Nos. 3,949,797 and 4,854,360 and French Patent Nos. 2,235,810and 2,472,484. The use of folded belts allows certain performanceimprovements, however, the cost of making tires with folded beltpackages is increased by their relatively complex constructionrequirements. Also, these tires have greater mass as a result of thefolded belt plies.

A number of patents illustrate only two unfolded belt plies in the beltpackage with one belt ply having aromatic polyamide as a reinforcingmaterial. French Patent No. 2,248,161 discloses a belt package with asingle outer belt ply having steel reinforcing members and a singleinner belt ply having aromatic polyamide (aramid) reinforcing members.The angles of the reinforcing members are briefly mentioned in thedisclosure as being "normal" values, without further definition. Thearamid belt ply is wider than the steel belt ply in this disclosure. InU.S. Pat. No. 4,936,366 the single unfolded aromatic polyamidereinforced belt ply can be the inner belt ply or the outer belt ply, andthe same is true with the other unfolded steel reinforced belt ply. Thispatent requires the angles of the steel reinforcing members to begreater than the angles of the aromatic polyamide reinforcing members by1 to 6 degrees. This angular relationship is specified to substantiallyequalize the bending stiffness between the two belt plies. Thisstiffness relationship should allow the two belt plies to beinterchanged. Angles of the steel reinforcing members are in a range of18 to 25 degrees where the angles of the aromatic polyamide reinforcingmembers are in a range of 17 to 24 degrees. The width of the inner beltply may be equal to the width of the outer belt ply with both equal toor slightly greater than the tread width. The disclosed breakingstrength of the 2×0.30 millimeter high strength steel reinforcingmembers is much lower than the disclosed braking strength of the 1670×3aromatic polyamide reinforcing members. Also, the pace (spacing) of thesteel reinforcing members in U.S. Pat. No. 4,936,366 is disclosed to beless than the pace of the aromatic polyamide reinforcing members.

In U.S. Pat. No. 3,851,693 all of the examples described in thespecification (no drawings) concern three belt plies consisting of twoconventional steel reinforced layers and one textile reinforced layer(rayon or aromatic polyamide). The steel reinforcing members are at 15degrees from a mid-plane and the textile reinforcing members are at 32degrees from this same plane. For future discussions in thisspecification the angles are assumed to be measured relative to themid-plane unless otherwise specified. A further discussion of the claimsof this patent follows.

Two patents illustrate belt packages with more than two belt plies butclaim a tire with only two belt plies. In U.S. Pat. No. 3,851,693 twosteel reinforced belt plies and one aromatic polyamide reinforced beltply are described and compared in the examples. In claim 1 only a singlesteel belt ply is claimed with steel reinforced belt members having anangle of 5 to 25 degrees and the other ply has aromatic polyamidereinforcing members having an angle of 0 to 45 degrees. Angles used inthe examples were 15 degrees for the steel members and 32 degrees forthe aromatic polyamide members. Specific values for the steel cabletwist coefficient, modulus of the reinforcing members and the modulus ofthe rubber at 300 percent elongation are claimed parameters to achievehigh speed performance. The tenacity, modulus and twisting coefficientfor the aromatic reinforcing members are also a part or this claimedinvention. Such material parameters are narrow in scope in comparison tothe very broad scope when using both material properties and stiffnessvariations which result from varying the number of the reinforcingmembers per unit width in each belt ply. Individual stiffness values orrelative stiffness properties between the two belt plies are not designparameters used for the tire of this reference.

Japanese Patent No. 2-136,304 also has only examples of tires with threebelt plies, including a cap ply having nylon reinforcing members. Thispatent claims a belt package with two plies; one being a single innerbelt ply reinforced with steel members and the other a single outeraromatic polyamide reinforced belt ply. The first angle of the steelreinforcing members is from 0 to 30 degrees and the second angle of thearomatic polyamide reinforcing members is set to be larger than thefirst angle and also in a range of 0 to 30 degrees. The angles used inan example are 22 degrees for the steel reinforcing members and 30degrees for the aromatic polyamide reinforcing members. The patent issilent in regard to the pace of the reinforcing members, but the pliesare designed such that the in-plane and out-of-plane stiffnesses of thearomatic polyamide reinforced belt ply are less the same stiffnesses ofthe steel reinforced belt ply that it replaces. To compensate for thisand maintain the in-plane bending rigidity of the belt package the angleof the steel reinforcing members is made less than the angle of thearomatic polyamide reinforcing members. The belt package stiffness alsorelies on the "triangular truss structure" made possible by the nyloncap ply used in the examples.

Whereas the art defines reinforcing member angles over a very wide rangeof values, no specific information is provided on the density of thebelt reinforcing members (pace) to achieve specific belt ply strengthand stiffness values and parametric relationships between the two plies.The need exists to define the structural parameters of a steelreinforced inner belt ply along with an aromatic polyamide reinforcedouter belt ply in sufficient detail such that the tire has less weightand continues to perform as well as or better than a tire with two steelreinforced belts. This includes defining the physical properties of thebelt reinforcing and rubber matrix materials as well as the density ofreinforcing members in each ply, their angles from the mid-plane of thetire and stiffness values for the two belt plies and the belt package.

SUMMARY OF THE INVENTION

An object of this invention is to define a low mass belt package for aradial pneumatic tire having an endurance equal to or greater than astandard low cost tire having a belt package with two metallicreinforced belt plies. The tire shall not have folded belts and capplies so that it can be economically manufactured.

Another object of this invention is to teach how to utilize the densityand angles of the reinforcing members for a metallic reinforced firstbelt ply and an aromatic polyamide reinforced second belt ply of thebelt package to yield a belt package having a balanced strength per unitwidth relationship between respective plies and stiffness values thatyield improved high impact endurance and corrosion performance.

A further objective of this invention is to achieve a low mass tirehaving an improved speed rating over prior art tires without degradationof the wear life of the tire.

In particular, this invention is a radial pneumatic tire comprising atoroidal carcass having at least one reinforced ply extending from afirst bead portion to an axially displaced second bead portion. The tirefurther comprises a circumferentially extending ground engaging treadportion disposed radially outward of a crown portion of the carcass. Asidewall portion extending radially inward from each axial edge of thetread portion to a respective bead portion; and a belt package betweenthe tread portion and the crown portion which has a predetermined widthand which consists of a single unfolded first belt ply havingessentially parallel metallic reinforcing members at a first angle fromthe mid-plane of the tire and is positioned radially outward of thecarcass, and a single unfolded second belt ply having essentiallyparallel aromatic polyamide reinforcing members at a second angle fromthe mid-plane of the tire, and positioned radially outward of the firstbelt ply. The belt package is further characterized in that the firstangle is less than the second angle by at least one degree wherein: (1)the breaking strength of each of the individual metallic reinforcingmembers is greater than a breaking strength of each of the individualaromatic polyamide reinforcing members; and (2) a strength per unitwidth of the second belt ply in a circumferential direction is withinzero to 30 percent greater than another strength per unit width of thefirst belt ply in the circumferential direction.

In another embodiment of this invention the pneumatic tire comprises atoroidal carcass having at least one reinforced ply extending from afirst bead portion to an axially displaced second bead portion; acircumferentially extending ground engaging tread portion disposedradially outward of a crown portion of the carcass. A sidewall portionextending radially inward from each axial edge of the tread portion to arespective bead portion; and a belt package between the tread portionand the crown portion having a predetermined width and which consists ofa single unfolded first belt ply having essentially parallel metallicreinforcing members at a first angle from the mid-plane of the tire andis positioned radially outward of the carcass, and which consists of asingle unfolded second belt ply having essentially parallel aromaticpolyamide reinforcing members at a second angle from the mid-plane ofthe tire and positioned radially outward of the first belt ply. The beltpackage is further characterized in that the first angle is less thanthe second angle by at least one degree wherein: (1) the breakingstrength of each of the individual metallic reinforcing members isgreater than the breaking strength of each of the individual aromaticpolyamide reinforcing members; and (2) the bending stiffness of thefirst belt ply about a lateral y-axis has a value in the range of about5 percent to about 150 percent greater than the bending stiffness of thesecond belt ply about the lateral y-axis where the lateral y-axis isparallel to a rotational axis of the tire.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention will become apparent to thoseskilled in the art to which the present invention relates from readingthe following specification with reference to the accompanying drawings,in which

FIG. 1 is a cross-sectional view of the tire showing a belt packagehaving two belt plies in accordance with this invention;

FIG. 2 is a plan view of the belt package of this invention showing thereinforcing members in each ply;

FIG. 3 is a graph showing the relative strength per unit width of thetwo plies of a belt package of one embodiment of this invention;

FIG. 4 is a graph showing the relative strength per unit width of thetwo plies of the preferred belt package of this invention;

FIG. 5 is a graph of actual vs. predicted high impact endurance valuesfor six test tires; and

FIG. 6 is a bar graph showing the bending stiffness of each belt ply forfour possible belt package configurations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The tire of this invention is a radial pneumatic tire suitable for useon a passenger car, sports car, light truck or like vehicle. Theobjective of a low cost and light weight tire is achieved by usingrelatively simple construction methods. No cap plies are used with thetire of this invention, and the carcass plies as well as the beadconstruction are standard in the industry. A cross-sectional view of thetire of this invention is illustrated in FIG. 1. The tire 10 maintainsits air pressure by having an innerliner portion 50.

The tire 10 of this invention as illustrated in FIG. 1 has a toroidalcross-section formed by a carcass. The toroidal carcass 30 has at leastone reinforced ply extending from a first bead portion 20 to an axiallydisplaced second bead portion 22. Each end 32 of the carcass 30 wrapsaround a respective bead 20 or 22 and extends radially outward from thebead. The radial direction is that associated with the z-axis directionextending outward from the bead toward a tread 60 of the tire. The tread60 has a tread width TW being a widthwise portion of the tire in thelateral y-axis direction that contacts a surface supporting the tire. Asidewall portion 40 extends radially inward from each lateral edge ofthe tread portion 60 to a respective bead 20 or 22.

A belt package 12 is located between the crown of the carcass and thetread as illustrated in FIG. 1. The belt package 12 has a lateral widthbeing a predetermined value consistent with the tread width TW. The beltpackage consists of a first belt ply 14 located radially outward of thecrown of the carcass and radially inward of the tread 60. The first beltply 14 preferably has a lateral width value in the range of about 100 toabout 105 percent of the tread width TW. The belt package 12 alsoconsists of a second belt ply 16 being radially outward of the firstbelt ply 14 and also radially inward of the tread 60. The second beltply preferably has a lateral width value of about 90 percent to about100 percent of the tread width TW. The first belt ply 14 is made to havea lateral width at least 5 percent greater than the lateral width of thesecond belt ply 16. An edge distance E between the inner belt ply 14 andthe outer belt ply 16 is shown in FIG. 2. The belt plies are symmetricalabout the mid-plane M and the edge distance E exists at both edges.

The construction of the belt package 12 is illustrated in FIG. 2, asviewed radially inward from the tread side of the tire. The first beltply 14 has metallic reinforcing members 15 disposed at a first angle A1from the mid-plane M of the tire. A first spacing or pace P1 betweenadjacent essentially parallel metallic reinforcing members 15 isexaggerated in this illustration to clearly show their orientation. Thebreaking strength of each individual reinforcing member is illustratedby force t1. The metallic reinforcing members 15 are preferably made ofsteel wires having two to four wires. Each wire can have a 0.23 to 0.32millimeter diameter and is made with carbon steel having about 0.70percent to about 0.80 percent of carbon by weight. The preferreddiameter is 0.28 millimeter. The steel reinforcing members are commonlydesignated in the industry as 2×0.28, 3×0.28 or 4×0.28 normal strengthsteel reinforcing members. A twist rate standard in the industry is alsoused. The preferred metallic reinforcing member for this invention isthe 4×0.28 member. The breaking strength t1 of the steel reinforcingmembers has a value in the range of about 30 deca Newtons (daN) to about65 daN. The tensile modulus of elasticity for the steel reinforcingmembers is in a range of about 120 giga Pascals (GPa) to about 185 GPa.The preferred tensile modulus of the metallic reinforcing member isapproximately 170 GPa. Other cables can be used within the scope of thisinvention including member designations 2×0.30 and 4×0.23. members. Anessential factor in this invention is the pace P1 of the steelreinforcing members to achieve predetermined strength and stiffnessvalues for the first belt ply 14 in relation to the second belt ply 16.A pace P1 in the range of about 1.00 millimeter to about 1.70 millimeteris anticipated. The preferred pace P1 of the steel reinforcing membersis about 1.60 millimeters for the preferred tire 10 of this invention.

The second belt ply has aromatic polyamide reinforcing members 17disposed at a second angle A2 from the mid-plane M, in a directionopposite to the first angle A1. A second spacing or pace P2 between theessentially parallel aromatic polyamide reinforcing members 17 isamplified to illustrate their relative orientation. The breakingstrength of the aromatic polyamide reinforcing members is illustrated bythe second force t2. The aromatic polyamide members preferably have atwo ply construction with a diameter in the range of about 0.55millimeter to about 0.70 millimeter. The aromatic polyamide reinforcingmembers 17 preferably have a mass per length of 1500 grams/9000 meters(denier), commonly referred to as 1500×2 denier cords. The breakingstrength t2 of the aromatic polyamide reinforcing members is preferablya value in the range of about 30 daN to about 50 daN. The pace P2 has avalue in the range of about 0.90 millimeter to about 1.45 millimeters.The preferred pace P2 of the aromatic polyamide reinforcing members hasa value of about 0.95 millimeters for the preferred tire 10 of thisinvention. A three ply cord construction, being 1500×3, is also withinthe scope of this invention. The tensile modulus of elasticity of thearomatic polyamide members, being a secant modulus at 1-2 percent unitstrain, is in the range of about 20 GPa to about 23 GPa. A ratio of thebreaking strengths t1/t2 for the preferred belt package reinforcingmembers 15 and 17 of this invention is preferably in a range of valuesof 1.05 to 1.75. That is,

    1.05<t1/t2<1.75

To balance the load taken by the respective belt plies 14,16 of the beltpackage 12 and produce a belt package having greater endurance, a "plystrength" or strength per unit width in the circumferential direction isspecified for the tire 10 of this invention. The strength per unit width(T) of a belt ply can be expressed by the equation: ##EQU1## where t isthe breaking strength of a respective reinforcing member, A is therespective angle between a longitudinal extent of the reinforcingmembers and a mid-plane M and P is a respective spacing of thereinforcing members. The strength per unit width is defined for both thefirst ply 14 and the second ply 16, and a system of equations can bewritten as follows: ##EQU2## There are three independent variables t, Pand A for each dependent variable T. Any one or all of the independentvariables can be changed to affect the strength per unit width T.Alternately, two or more independent variables can be changed withoutchanging the dependent variable T (i.e., the angle A can increase alongwith a decrease in spacing P to give the same T). The ability to changeT1 and T2 by changing either respective angles A1 and A2, respectivebreaking strengths t1 and t2 or respective spacings P1 and P2 isessential to be able to adjust the relative value of T1 with respect toT2 for the different reinforcing members of this invention. Theindependent variables are illustrated in FIG. 2. The improved beltpackage is characterized by a relative strength per unit width betweeneach respective belt to yield a balanced design. That is, defining therelationship of the dependent variable T1 with respect to dependentvariable T2 is essential to achieve the objectives of this invention.Preferably, the relative strengths per unit width are such that:

    T1<T2<1.3×T1

within the range of angles A1,A2 given herein for each ply. Bydefinition, these values of T1 and T2 are also a function of thebreaking strengths t1,t2 and the reinforcing member spacings P1,P2 asdefined herein.

The angles A1 and A2 of the reinforcing members from the mid-plane M areselected to help give relative strength per unit width values withindefined limits. The angles can be easily changed to vary strength perunit width values of each belt ply to achieve a balanced strength perunit width relative value between the two belt plies. A balanced designbased on relative strength per unit width values between the two beltplies (T1 vs. T2) is achieved by having a predetermined value ofstrength per unit width known to give improved tire endurance. It hasbeen determined that a T1 value of approximately 350 deca Newtons percentimeter gives a well balanced design.

Having selected the reinforcing members with breaking strengths in therange of values disclosed herein, the angle A of the reinforcing membersis selected for each belt ply 14,16 to best conform to low costmanufacturing, and to be within a reasonable range of values to keep themembers in tension. Belt ply reinforcing members have much moreendurance in resisting tensile forces. A large angle range, as taught bythe art, is not practicable. The steel reinforcing members 15 arepositioned so that the first angle A1 with respect to the mid-plane M isin a range of values of about 20 degrees to about 28 degrees. Thearomatic polyamide reinforcing members 17 are positioned such that thesecond angle A2 from the mid-plane M is in a range of values of about 22degrees to about 29 degrees.

The pace P of reinforcing members in each of the belt plies is adjustedto substantially yield a preferred strength per unit width value.Preferred values for the first strength per unit width T1 are in a rangeof values from about 275 deca Newtons per centimeter (daN/cm) to about400 daN/cm. For a balanced design the relative strength per unit widthvalues (T1 vs. T2) are selected such that a second strength per unitwidth value T2 of the second belt ply 16 in a circumferential directionis within zero to 30 percent greater than the first strength per unitwidth value T1 of the first belt ply 14 in the same circumferentialdirection. The preferred second strength per unit width T2 is in a rangeof values from about 390 daN/cm to about 450 daN/cm. For example, thegraphs of FIGS. 3 and 4 show relative strength per unit width valueswithin 30 percent of each other for a belt package. In this example, thevalues of breaking strength t are known using the reinforcing members ofthis invention and the pace P has been selected to make T2 within zeroto 30 percent greater than the value of T1 over a range of preferredangular values A1 and A2.

The graphs of FIGS. 3 and 4 further show what the effect of changing theangle A of the reinforcing members has on the strength per unit width Tvalues. The strength per unit width is simply called "strength" in thegraphs. The curve 71 of FIG. 3 shows the value of the first strength perunit width T1 of the first belt ply 14 in the circumferential directionas a function of the angle A1 using 3×0.28 steel reinforcing members.The curve 81 of FIG. 4 shows the same first strength per unit width T1as a function of the angle A1 using the preferred 4×0.028 reinforcingmembers. The curve 72 of both FIG. 3 and FIG. 4 shows the value of thesecond strength per unit width T2 of the second belt ply 16 in thecircumferential direction as a function of the angle A2 of the preferred1500×2 aromatic polyamide reinforcing members. To make the strength perunit width equal in both belt plies in FIG. 4, select an angle A1 ofabout 21 degrees for the steel reinforcing members and project thevertical line 84 to the curve 81. A horizontal line 86 corresponding toa value of 350 deca Newtons per centimeter (daN/cm) is projected tocurve 72 for the aromatic polyamide reinforcing member. An intersectionwith curve 72 is projected vertically by line 88 to give an angle A2 ofabout 27 degrees for the aromatic polyamide reinforcing members.Therefore, for this example, an angle difference of six degrees willgive a completely balanced design by having equal belt ply stiffness perunit width values for the two belt plies 14,16. Once again, thepreferred second strength per unit width T2 of the second belt ply in acircumferential direction is within zero to 30 percent greater than thefirst strength per unit width T1 of the first belt ply in the samecircumferential direction.

In another embodiment of this invention the same tire 10 is further madeto have a high impact endurance. This type of endurance is associatedwith a tire that is resistant to shock loading from road hazards, bumps,pot holes and the like in a roadway. These loading conditions greatlyreduce the overall endurance of the tire and have been found to beassociated with certain stiffness values of the individual belts, aswell as the composite belt package. There are five different stiffnessvalues that characterize each belt plus the belt package of the tire.These values can be calculated for individual belt plies as well as forthe total belt package. Two values are extensional stiffness values, twoare bending stiffness values and one is a shear stiffness value. Methodsto calculate these stiffness values for composite materials are wellknown in the art. The mathematics is outlined by the matrix equations in"Mechanics of Composite Materials" by Robert M. Jones, which waspublished by Hemisphere Publishing Corp. in 1975. In particular, thesematrix equations are written in terms of the middle surface extensionalstrains and curvatures as summarized in section 4.4.1 of this reference.The method is well known in the tire industry where bending stiffness isdefined using beam theory over a unit width of the belt ply and shearstiffness is defined according to plate theory as being proportional tothe cube of the belt ply thickness. This material is included herein byreference.

High impact endurance HIE was found to have a direct correlation withthe relative bending stiffness about the lateral y-axis (FIG. 2) betweenindividual belt plies, as well as the composite shear stiffness of thebelt package in the x-y plane. The relationship for high impactendurance (HIE) as a function of stiffness values, was determined to be

    HIE=FUN (BY1-BY2), CGZ!

where the independent variables are BY1-BY2 and CGZ. The parameter BY1is the bending stiffness of the first belt ply about the lateral y-axis,parameter BY2 is the bending stiffness of the second belt ply about thelateral y-axis and parameter CGZ is the composite shear stiffness of thebelt package in the x-y plane (about the z-axis). The value of HIE isthe distance traveled by the tire during a high impact endurance testbefore it fails. The HIE is directly proportional to the difference inbending stiffness values BY1-BY2 as well as the composite shearstiffness CGZ. Proportionality coefficients can be determined from acorrelation analysis for each tire size and construction using highimpact endurance test values. For example, FIG. 5 shows a 1:1 curve 90for an exact correlation of predicted vs. actual HIE values; with theresults of six test tires 3A, 3B, 3C, 4B, 5A and 5B plotted relative tothe exact correlation curve. The high impact endurance magnitudes shownare relative to tire 4B having a value of 100. Proportionalitycoefficients were obtained for this test series of six tires. Thestandard R×R confidence factor was 0.97 for this high impact testexample; being an excellent correlation of actual test data with thatpredicted by the functional relationship for HIE. The relative magnitudeof the high impact endurance values are accurately predicted by the HIEfunctional relationship.

From the high impact endurance functional relationshipHIE=f(BY1,BY2,CGZ) above, it is noted that improved endurance existswhen the bending stiffness difference BY2-BY1 is negative; such thatthis term increases the magnitude of HIE. That is, the bending stiffnessBY1 of the first belt ply about the lateral y-axis should be madegreater than the bending stiffness BY2 of the second belt ply about thesame lateral y-axis. Optimum values for BY2-BY1 with respect to CGZ canbe achieved to maximize the high impact endurance HIE value. Values ofbending stiffness BY1 in the range of about 5 percent to about 150percent greater than the bending stiffness BY2 have been determined asoptimum for various tire designs. Preferably the bending stiffness BY1of the first belt ply is at least 45 percent greater than the bendingstiffness BY2 of the second belt ply about respective lateral y-axes.The relative magnitude of the bending stiffness BY2-BY1 is an essentialfactor for improved high impact endurance, while maintaining arelatively high composite shear stiffness CGZ value. The preferredcomposite shear stiffness value is greater than about 2500 mega Pascalmillimeters.

The bar graph of FIG. 6 illustrates the relative bending stiffnessBY2-BY1 of belt plies in four test tires 3C, 3A, TT1 and TT2 havingdifferent belt package constructions. The same steel reinforcing members(4×0.28) having the same pace (1.60 millimeters) is used in each firstbelt ply of each tire and the same aromatic polyamide reinforcingmembers (1500×2) having the same pace (0.94 millimeter) is used in eachsecond belt ply of each tire. The left vertical bar of each set of barsis for the first belt ply 14 and the vertical bar on the right is forthe second belt ply 16 of the belt package 12. The tires 3C and 3A arethe same tires as illustrated in FIG. 5. The bar designated S24 is for asteel reinforced first belt ply 14 with reinforcing members at 24degrees from the mid-plane M of the tire 10. The bar designated A28 isfor a aromatic polyamide reinforced second belt ply 16 with reinforcingmembers at 28 degrees from the mid-plane M. Relative values of thebending stiffness, called "stiffness" in the graph, are given based on arange of values from 0 to 100. Note that a much greater relative bendingstiffness BY2-BY1 is possible using the test tire TT2 of this invention;where the steel reinforcing members are at 24 degrees and the aromaticpolyamide reinforcing members are at 28 degrees.

The high impact endurance HIE functional relationship also shows acorrelation with composite shear stiffness CGZ about the z-axis. A tire10 having a higher composite shear stiffness will yield a tire having alarger high impact endurance when all other factors are the same. Tire3A has the same relative bending stiffness BY2-BY1 as tire 3C in FIG. 6,however, the composite shear stiffness of tire 3A is greater than thatof tire 3C such that the overall high impact endurance HIE of tire 3A isgreater than that of tire 3C (see FIG. 5).

The parameters that yield a balanced strength per unit widthrelationship also result in a higher impact endurance. It is essentialto have the angle A1 of the steel reinforcing members less than theangle A2 of the aromatic polyamide reinforcing materials for thepreferred reinforcing materials of this invention. Angular differencesA2-A1 can be in a range of values from about 1 degrees to about 8degrees. The preferred difference A2-A1 is approximately 3 degrees.

The stiffness of the belt plies are not only a function of the tensilemodulus of the reinforcing members, the spacing or pace P of thereinforcing members and the angle A of reinforcing members from themid-plane M, but also the thickness and material properties of therubber matrix material within each belt ply. The modulus of elasticityof the rubber matrix material 18,19 (FIG. 2) at ten percent unit strainis in a range of values of about 5 mega Pascals (MPa) to about 25 MPa.The preferred rubber modulus of elasticity at ten percent unit strain is11 MPa for the tires 10 of this invention. The thickness of the greenrubber skim on each side of each of the reinforcing members of the beltpackage has a value in the range of about 0.35 millimeter to about 0.90millimeter. The green rubber skim thickness on each side of the steelreinforcing members 15 is preferably 0.65 millimeters and the greenrubber skim thickness on each side of the aromatic polyamide reinforcingmembers 17 is preferably 0.40 millimeters. Other skim thickness valuesare also possible within the scope of this invention. Rubber materialproperties and skim thickness values are selected to give the low costlight weight durable tire of this invention.

EXPERIMENTAL EVIDENCE

The tire of this invention has a balanced strength per unit widthrelationship between each belt ply in the circumferential direction, alarge difference in the bending stiffness about a lateral y-axis betweenthe two belt plies and a relatively large composite shear stiffness ofthe belt package in the plane of the belt plies. Experimental evidencehas shown that this tire 10 has improved running endurance as well ashigh impact endurance. The tire has a relatively low mass by using asecond belt with aromatic polyamide reinforcing members. In addition,the reduced pace of the aromatic polyamide reinforcing members is suchthat corrosion of the first steel reinforced belt ply is reduced.

A passenger car tire having a first ply with steel reinforcing membersand a second ply with aromatic polyamide reinforcing members (aramidcords) was compared with a witness tire having both plies with steelreinforcing members. Both tires were of the same P185/65 R14 size andconstruction. The steel reinforced belt plies were all reinforced with4×0.28 steel reinforcing members at an angle A1 of 20 degrees and thearomatic polyamide reinforced belt ply was reinforced with 1500/2aromatic polyamide reinforcing members also at an angle A2 of 23degrees. The pace of the steel members was 1.60 millimeters and the paceP2 of the aromatic polyamide reinforcing members was at 0.95millimeters. The following table shows a comparison of physicalparameters.

    ______________________________________                              STEEL-ARAMID TIRE                 WITNESS TIRE steel reinf. 1st ply                 steel reinf. 1st ply                              aromatic polyamide    PARAMETER    steel reinf. 2nd ply                              reinf. 2nd ply    ______________________________________    MASS         7.1          6.8    (kilograms)    HIGH SPEED LIMIT                 100 (ref.)   114    (relative)    UTQG TEMPERATURE                 100 (ref.)   108    (relative)    WEAR RATE    100 (ref.)   75    most worn rib    (relative)    ______________________________________

Other performance parameters were essentially the same in this testcomparison. Handling and comfort of the tires were also approximatelyequal. Further parametric improvements were obtained when the aromaticpolyamide reinforcing members of the second belt ply were disposed at anangle A2 from the mid-plane between 1 to 8 degrees larger than thedisposed angle A1 of the steel reinforcing members of the first beltply. An improvement in the high impact endurance HIE of tire TT2 of FIG.6 over the above witness tire is anticipated to be at least 10 percent.

Consider the two tires TT1 and TT2 of FIG. 6 as having the sameconstruction except for the angle A1 of the steel reinforcing membersrelative to the angle A2 of the aromatic polyamide reinforcing members.The tire TT1 with A1>A2 has a composite shear modulus CGZ 4 percentgreater than tire TT2 with A1<A2. However, tire TT2 has a calculatedhigh impact endurance HIE which is 6.2 percent greater than the HIE fortire TT1.

From the above description of the preferred embodiments of theinvention, those skilled in the art will perceive improvements, changesand modifications. Such improvements, changes and modifications withinthe skill of the art are intended to be covered by the appended claims.

Having described the embodiments of the invention what is claimed is: 1.A radial pneumatic tire comprising:a toroidal carcass having at leastone reinforced ply extending from a first bead portion to an axiallydisplaced second bead portion; a circumferentially extending groundengaging tread portion disposed radially outward of a crown portion ofthe carcass; a sidewall portion extending radially inward from eachaxial edge of the tread portion to a respective bead portion; and a beltpackage between and adjacent both the tread portion and the crownportion of the carcass and having a predetermined width and consistingof a single unfolded first belt ply having essentially parallel metallicreinforcing members at a first angle from a mid-plane and positionedradially outward of the carcass and a single unfolded second belt plyhaving essentially parallel aromatic polyamide reinforcing members at asecond angle from the mid-plane and positioned radially outward of thefirst belt ply, said first angle being less than said second angle by atleast one degree, wherein (1) the breaking strength of each of theindividual metallic reinforcing members is greater than the breakingstrength of each of the individual aromatic polyamide reinforcingmembers, and (2) a strength per unit width of the second belt ply in acircumferential direction is within zero to 30 percent greater than astrength per unit width of the first belt ply in the circumferentialdirection.
 2. The tire set forth in claim 1, wherein a ratio of thebreaking strength of each of the individual metallic reinforcing membersof the first belt ply to the breaking strength of each of the individualaromatic polyamide reinforcing members of the second belt ply has avalue in the range of 1.05 to 1.75.
 3. The tire set forth in claim 1,wherein the predetermined width of the belt package is characterized bya first width of said first belt ply being in a range of about 100 toabout 105 percent of a tread width of the tire and a second width ofsaid second belt ply being in a range of about 90 percent to about 100percent of the tread width, said first width being greater than saidsecond width by at least 5 percent of the tread width.
 4. The tire setforth in claim 1, wherein the metallic reinforcing members are made ofmultiple strands of steel wire having a carbon content in a range ofabout 0.7 percent to about 0.8 percent by weight to give a tensilemodulus of elasticity of each metallic reinforcing member in a range ofabout 120 giga Pascals (GPa) to about 185 GPa.
 5. The tire set forth inclaim 1, wherein the aromatic polyamide reinforcing members have a twoply construction and a diameter in the range of about 0.55 millimetersto about 0.70 millimeters and a secant modulus at 1 to 2 percent strainin the range of about 20 giga Pascals (Gpa) to about 23 GPa.
 6. The tireset forth in claim 1, wherein said metallic reinforcing members arepositioned such that the first angle is in a range of about 20 degreesto about 28 degrees and said aromatic polyamide reinforcing members arepositioned such that the second angle is in a range of about 22 degreesto about 29 degrees, said first angle being opposite to said secondangle with respect to the mid-plane.
 7. The tire set forth in claim 1,wherein the breaking strength of the individual metallic reinforcingmembers has a value in the range of about 30 deca Newtons (daN) to about65 daN and the breaking strength of the individual aromatic polyamidereinforcing members has a value in the range of about 30 daN to about 50daN.
 8. The tire set forth in claim 1, wherein said belt package isfurther characterized by a composite shear stiffness greater than about2500 mega Pascal millimeters.